ON THE ORIGIN OF ALKALINE NEPHELINE-BEARING ROCKS OF THE TYRDANOV ULUS AREA

(Kuznetsk Alatau)

A. A. MITYAKIN

(Presented by the scientific seminar of the Department of Petrography)

The area is located in the middle course of the Sukhaya Erba River, on the right side of the valley. Geologically, the area belongs to the endocontact zone of the northern termination of the Uybat granitoid pluton of varied composition. The endocontact zone of the pluton is composed of rocks of varied composition, ranging from syenites and diorites to hornblendites, and sometimes pyroxenites, with various transitional rocks between them.

The first information on the alkaline rocks of the area was published in the early 1940s by Yu. A. Kuznetsov. He regarded them as products of the reaction interaction of a magmatic melt of acid composition with enclosing carbonate rocks, mainly of Proterozoic age, represented by limestones and dolomites. Subsequently, however, many researchers separated the alkaline rocks from the Uybat pluton and considered them to be the product of a later Devonian intrusion. This same viewpoint was also expressed in one of the recent monographs on alkaline magmatism of the Kuznetsk Alatau (Andreeva, 1968). The author of the monograph considers the alkaline rocks of Tyrdanov Ulus to be a constituent part of the Devonian gabbro–syenite formation of the Kuznetsk Alatau. Studies carried out by the present author make it possible to conclude the following: the nepheline rocks of the area are rather diverse in composition; they are represented by nepheline syenites, nepheline diorites, nepheline monzonites, and other varieties transitional between them.

Macroscopically, the nepheline-bearing rocks are often very similar to the corresponding nepheline-free rocks developed in the endocontact zone of the Uybat pluton. One can observe how nepheline-bearing rocks gradually pass into nepheline-free rocks, and then into quartz-bearing rocks. The principal rock-forming minerals in the nepheline-bearing and nepheline-free rocks are the same. This is usually common hornblende with angles \(CNg = 16—23^\circ\), \(2V = -72—81^\circ\). Recalculation of the chemical analysis of hornblende gives the following formula:

\[ (\mathrm{Ca}_{1.89}\mathrm{Na}_{0.67}\mathrm{K}_{0.28})_{2.84} (\mathrm{Fe}^{\cdot\cdot}_{1.23}\mathrm{Fe}^{\cdot\cdot\cdot}_{1.07}\mathrm{Mg}_{1.89})_{4.19} \mathrm{Al}_{2} (\mathrm{Si}_{5.60}\mathrm{Ti}_{0.49})_{6.09} (\mathrm{O}, \mathrm{OH})_{22}. \]

In the nepheline-bearing rocks, in addition to common hornblende, an alkaline amphibole—arfvedsonite—is observed, with optical constants \(CNp = 32^\circ\), \(2V = -80—100^\circ\), \(Ng = 1.668\), \(Np = 1.652\), \(Ng - Np = 0.016\).

The potassium feldspar in the nepheline-bearing and nepheline-free rocks is microcline with an angle \(2V = -76—88^\circ\); according to its chemistry it contains ...

contains 45.5% of the albite component. Plagioclase is most often oligoclase from No. 13 to No. 29; sometimes its basicity increases to Nos. 40–45.

Attention is drawn to the wide development of pyroxene both in nepheline-bearing and in nepheline-free rocks. Pyroxene is observed even in quartz diorites. According to its optical constants, this is augite; in nepheline-free rocks it is sometimes colorless to pale green, while in nepheline-bearing rocks a denser green coloration along Ng to greenish-yellow along Np is observed; the angle CNg ranges from 40 to 54°, \(2V = +60—73^\circ\). Green pyroxene from melanocratic nepheline-free diorite, with constants \(CNg = 48^\circ\), \(2V = 65^\circ\), according to chemical analysis has the formula:

\[ (\mathrm{Ca}_{1.05}\mathrm{Mg}_{0.32}\mathrm{Fe}_{0.36}\mathrm{Fe}_{0.14}^{\cdots}\mathrm{Ti}_{0.05}\mathrm{Na}_{0.07}\mathrm{K}_{0.01})_{2.10}(\mathrm{Si}_{1.63}\mathrm{Al}_{0.35})_{1.98}\mathrm{O}_6. \]

Noteworthy is the high calcium content in the pyroxene—26%. Among nepheline-bearing rocks, garnet-bearing nepheline syenites are observed; the garnet content sometimes reaches 20%. Nepheline-free garnet rocks are also observed. Under the microscope, the garnet is dark-colored, with resorbed edges, and is often found intergrown with nepheline.

According to chemical analysis, the garnet has the formula

\[ (\mathrm{Ca}_{2.68}\mathrm{Ti}_{0.06}\mathrm{Fe}_{0.09}^{\cdots}\mathrm{Mn}_{0.04}\mathrm{Na}_{0.04}\mathrm{K}_{0.12})_{3.03}(\mathrm{Fe}_{1.16}^{\cdots}\mathrm{Al}_{0.82})_{1.98}[(\mathrm{Si}_{2.92}\mathrm{Ti}_{0.08})_3\mathrm{O}_{12}], \]

which corresponds to andradite. The calcium oxide content in the mineral is 30.73%. Among the nepheline-bearing rocks, in direct contact with the enclosing dolomites, nepheline pyroxenites were found. These are medium-grained rocks with the following mineralogical composition: pyroxene 90%, nepheline—5%, calcite—3%, apatite—2%.

Pyroxene, as in the other rock varieties, is green augite in the form of short-prismatic crystals. Nepheline is represented by crystals of isometric form, partly replaced by cancrinite. Calcite occurs as large, slightly elongated crystals with regular boundaries.

In addition to the noted features of mineralogical composition, geochemical features also indicate a genetic relationship between the nepheline-bearing and nepheline-free rocks. In both the former and the latter the same trace elements are contained in equal amounts (table).

Of the elements characteristic of alkaline rocks, only three are present: zirconium, beryllium, yttrium. At the same time the beryllium content is very close to the clarke value for acidic rocks and above the clarke value for basic rocks. Yttrium and zirconium are contained in amounts below clarke values for both acidic and basic rocks.

In nepheline-bearing and nepheline-free rocks, lead and barium, characteristic of acidic rocks, are present in equal amounts. The contents of these elements are close to clarke values for acidic rocks.

Chromium, cobalt, and nickel, characteristic of basic rocks, are contained in both groups of rocks under consideration in amounts significantly less than clarke values for basic rocks, but close to clarkes for acidic rocks.

On the basis of the foregoing, the following conclusion may be drawn: the observed gradual transitions from nepheline-bearing rocks to nepheline-free rocks of the endocontact zone of the Uibat pluton, the similar mineralogical composition, the wide development of pyroxene in both groups of rocks with elevated calcium content, the frequent presence in the rocks of garnet-andradite, the content of which in some nepheline rocks reaches 20%, and the presence at the contact with carbonate

Table 3

Content of trace elements in nepheline-bearing rocks of the Tyrdanov Ulus and in nepheline-free rocks of the endocontact zone of the Uibat pluton

rocks of the nepheline pyroxenites with relict carbonate and, finally, the similar geochemical features of both groups of rocks—all this indicates that the nepheline-bearing rocks do not constitute an independent intrusive body, but are genetically related to the nepheline-free rocks of the endocontact zone of the Uibat pluton. Both groups of rocks are the result of the reaction interaction of a granitic melt with the enclosing carbonate rocks, as a result of which desilication occurred at the site of formation of the granitic batholith.

The increase in alkalinity in the endocontact zone, with the formation of feldspathoid rocks, is explained by Korzhinskii’s well-known theory, according to which, when an acidic magma interacts with salts of strong bases and weak acids, such as carbonates, the activity of all bases increases, but especially that of strong bases such as Na₂O and K₂O, which leads to the accumulation of these bases within the endocontact zone.